The Internet, which is conventionally used simply as a tool for data transmission, for example, for accessing websites or transmitting electronic mails, is recently applied also for voice communication services, which require a high communication quality for data transmission and a high real-time property. Among such voice communication services, VoIP (Voice over Internet Protocol) systems including IP (Internet Protocol) phone systems using Internet technologies have been put into practice along with the spread of the broadband.
VoIP is a transmission technology for transmitting and receiving voices via an IP network by use of the Internet Protocol (IP). A representative example of VoIP is an IP phone technology. An IP phone service provides voice communication using an IP network of a communication carrier. Communication carriers each have an IP network configured exclusively for the IP phone service in order to keep high quality of the IP phone service. As described in, for example, Japanese Laid-Open Patent Publication No. 2009-272668, a user of an IP phone is assigned a phone number (e.g., number starting with “050”) for the IP phone by a communication carrier of the IP phone service. Such a user can perform voice transmission and receiving with another IP phone as well as with a landline phone using an existing public switched telephone network (PSTN).
However, with such an IP phone service, information on a user who transmitted a conversation request signal (such a user will be referred to as a “transmitting user”) may not be notified correctly to a user on the receiving side (such a user will be referred to as a “receiving user”) depending on the standards of the communication carriers or the specifications of the IP phone terminal which transmits or receives the signal. In such a case, even when a communication terminal of the receiving user (receiving terminal) is called in response to the request signal, the transmitting user information on a terminal of the transmitting user (transmission terminal) is not notified to the receiving terminal. As a result, the receiving terminal cannot make a conversation with the transmission terminal; or the receiving user, when not being aware of the call, does not learn who transmitted the conversation request.